I have a down stairs toilet connected to waste as usual. In addition there is a hand basin whose waste is connected via a P trap to the same waste pipe. Lately the water level in the toilet bowl falls allowing gas from the drains to enter the room. I have noticed that the water level in the bowl is oscillating after flushing. I propose to replace the P trap with an anti vac type. Is this the right thing to do? Thank you

Hi Sapper

If this is something that didn't happen until just now then replacing the trap is probably not going to resolve the issue... ;)

More likely here is that the toilet is partially clogged and siphoning the toilet water out, or the toilet/hand basin vent may be clogged.

I'd check this stuff before replacing any parts!

Back to you...

Mark

Not likely. If water is being siphoned out of the toilet bowl it is most likely because you have a blocked vent stack. Never heard of an anti vac P trap.

More details please! Does the bowl filln bup and slowly drain away. Is the level low from the git-go? Have you checked the little tube running from theballcock to the overflow tube? ( That's the one that fills the bowl. )
I have noticed that the water level in the bowl is oscillating after flushing Please explain what you mean. Describe this "oscillating". Back to you, Tom

Thanks to all who replied to me.
I believe the problem is resolved. I fitted the anti-vac P trap (available from Pochin) and the water level is now stable, at the correct level and is not oscillating after a flush. I have also used two buckets of water poured slowly into the loo and all appears well. Thanks so much chaps...

We thank you for the update. If we can help in the future please calln on us. Regards, n bnTom

This sounds like what is know here as a AAV air admittance valve. It made this issue go awy for now but the real problem is still there, most likely a clogged vent stack like Harold mentioned. Do not know what Pochin is but suspect you are not located in the USA, where are you living please

Like Harold and MARK mentioned, Bob... :p

I just wanted to get the true name for the AAV out there because anti vac P trap is not part of our vocab. Also reenforcing that the guys are correct. Me, I'd be fishing out that stack if I could avoid falling off my 12/12 roof.